1. Field of the Invention
This invention relates to a thread feeding device in a feeding control apparatus for a plurality of threads in a sewing machine. The apparatus supplies the threads consumed for forming stitches by thread feeding devices that are rotated forcedly without relying on the feeding operation of a thread take-up lever and uses the take-up lever only for the purpose of pulling the threads to tighten the stitch.
2. Description of the Prior Art
Since the present invention is primarily concerned with a thread feeding apparatus in a sewing machine using a plurality of threads arranged in such a manner that a necessary quantity of thread is fed out by a roller rotation feeding mechanism, the construction of a thread feeding mechanism in the sewing machine will be described first of all.
FIGS. 3 to 7 of the accompanying drawings show a sewing machine disclosed in Japanese Patent LaidOpen No. 279294/1986 and the thread feeding mechanism in this sewing machine will be described by way of example. FIG. 3 is a perspective view showing the sewing machine using a thread feeding control device by a roller, FIG. 4 is an explanatory view showing the construction of the thread feeding control device of FIG. 3, FIGS. 5(a) and 5(b) show an encoder mounted to a main drive shaft of the sewing machine, wherein 5(a) is a sectional side view and 5(b) is a front view of a slit disc, FIG. 6 is a sectional view of an electromagnetic thread holding device and FIGS. 7(a), (b) and (c) are diagrams showing the thread feeding control state, wherein 7(a) is an operation diagram of the electromagnetic thread holding device under the thread feeding state, 7(b) is its operation diagram under the high speed operation state of the sewing machine and 7(c) is its operation diagram under the low speed operation state of the sewing machine.
FIG. 3 is a perspective view showing a sewing machine A equipped with a thread feeding control device by a roller. As shown in FIG. 4, the sewing machine includes a needle 1, a thread take-up lever 2, a thread guide spring 3, thread guides 4, 5, 6, a thread holding device 7 on the side cf the thread take-up lever, a feed roller 8 equipped with an encoder which is driven by a motor rotating independently of the rotation of the main drive shaft of the sewing machine, a pressure roller 10 which is brought into or out of contact with or from the feed roller 8 by an electromagnetic controller 9, a thread holding device 11a, 11b on the thread feed side, a basic tension disc 12a, 12b and spools 13a, 13b. A rotary encoder 17 is fitted to the main drive shaft 16 equipped with a pulley 15 inside the sewing machine A. In the drawing, reference numerals 14a, 14b represent a thread and symbol B shows the portion of the thread feeding control device. The basic tension disc 12a, 12b need not always be contained in this portion.
The rotary encoder 17 is used to detect the end of the pulling operation of the thread to tighten the stitch by the thread take-up lever and a position sensor for the thread take-up lever used in a so-called "electronic sewing machine" can be used as this rotary encoder.
Tension is applied by the basic tension disc 12a, 12b to the thread 14a, 14b on the side of the spool 13a, 13b to such an extent as not to generate disturbance of the thread. The thread 14a, 14b is sent through the electromagnetic thread holding device 11a, 11b on the thread supply side which is under the inoperative state to the take-up lever side while being gripped between the feed roller 8 equipped with the motor driving encoder and the pressure roller 10. The feed control of the thread is effected in such a manner that the electromagnetic thread holding device 11a, 11b clamps the thread to stop its travel and to cause the slip of the thread over the feed roller 8 and the feed of the thread is thus stopped though the feed roller 8 keeps rotating. The electromagnetic controller 9 is disposed in order to release the pressing operation of the pressure roller 10 to the feed roller 8, whenever necessary.
As shown in FIGS. 7(b) and 7(c), the thread holding device 7 on the side of the take-up lever is equipped with the thread guide spring 3 in order to absorb the increase in the tension of the thread that occurs when the loop of the thread is passed around a rotating hook bobbin case holder by a rotating hook, when the thread passes between the rotating hook bobbin case holder and the tip of a rotation restraining finger or when the thread slips away from the tail of the rotating hook in addition to when the take-up lever pulls the thread to tighten the stitch. However, the thread guide spring may not be able to absorb the increase of tension in some structure or sewing conditions so that the tension acts on the thread which is being gripped and fed by the feed roller 8 and the pressure roller 10 and feeds the thread more than permitted by the rotation feeding operation of the feed roller 8. Accordingly, the thread is gripped at this time by the electromagnetic thread holding device 7 on the side of the thread take-up lever and any adverse influences on the thread feed action by the feed roller due to the change of tension are eliminated.
FIG. 5 shows an example of the rotary encoder 17 disposed inside the sewing machine. Photo cells 19 and 20 are disposed in a casing 18, through which the main drive shaft 16 penetrates, in such a manner as to face each other with a slit disc 21 between them. The end timing of the thread pulling operation by the thread take-up lever to tighten the stitch is detected by a signal from such an encoder to release the grip of the thread and to let the feed roller 8, equipped with a constant speed motor driving encoder, feed the thread.
FIG. 6 is a sectional view showing the structure of the electromagnetic thread holding devices 7 and 11a, 11b. A friction plate 25 is disposed on the upper surface of a coil spool 24 which comes into sliding contact with a cylindrical yoke 23 equipped with a magnet 22 and a cap 26 is disposed at the upper part of the yoke 23 so that the thread 14a, 14b can be gripped between the friction plate 25 and the back of the cap 26, and the feed operation of the thread by the feed roller 8 can be stopped. This operation can be made rapidly with hardly any inertia by passing a current through the coil spool.
Next, the operation of the thread feeding control device will be explained with reference to the diagrams shown in FIGS. 7(a), 7(b) and 7(c).
In the diagrams 7(b) and 7(c), symbol T.sub.1 represents the timing of generation of the thread tension due to the thread pulling to tighten the stitch by the take-up lever, T.sub.2 represents the upper dead-end of the take-up lever and T.sub.3 represents the timing of disappearance of the thread tension. As shown in the diagram 7(a), the feed of the thread by the thread feed roller 8 equipped with the encoder driven by a motor rotating at a constant speed is made by detecting the timing P (which is at the same time as, or a little bit after, the timing T.sub.3) by the signal from the rotary encoder 17 disposed on the main drive shaft of the sewing machine to release the grip of the thread by the electromagnetic thread holding device 11 on the thread supply side, then pushing the pressure roller 10 to the feed roller 8 to deliver the thread, operating again the electromagnetic thread holding device 11 at the time of completion (timing T.sub.4) of the feed of the length l of the thread necessary for one stitch by the feed quantity detection encoder disposed coaxially so as to grip the thread and then stopping the thread feed operation by the thread feed roller 8.
The thread feed roller 8 equipped with the encoder is constructed in such a manner as to rotate at a constant speed although the rotating speed of the sewing machine can be changed. Therefore, the same quantity of the thread is fed within the same period of time. This means that the thread is always fed in the length l between P and Q as shown in the diagram 7(a) and the relationship between the positions of P, Q and the curve of the thread take-up lever changes as shown in the diagrams 7(b) and 7(c), respectively, at high and low speed operations of the sewing machine. Needless to say, the length l of the thread necessary for one stitch can be adjusted by changing the count number of pulses of the encoder but construction must be made so that the maximum quantity necessary for one stitch can also be supplied during the period from the tension disappearance timing T.sub.3 to the next tension generation timing T.sub.1 during the high speed operation of the sewing machine. However, if an attempt is made to secure the longest possible period in which the thread can be supplied, tension unavoidably develops when the thread passes between the rotating hook bobbin case holder and the tip of the rotation restraining finger and the thread feed control by the feed roller 8 might be disturbed. Accordingly, the thread is gripped by another electromagnetic thread holding device 7 disposed closer to the thread take-up lever than the feed roller 8 during such a tension generation period, that is, from near the lower dead-end of the thread take-up lever up to the generation of thread pulling tension to tighten the stitch by the take-up lever. In order to prevent any force from acting on the feed roller 8 during its feeding operation, the grip of the thread by the thread holding device 7 is released in the period after the end of the feed operation and before the start of the next thread pulling operation by the take-up lever to tighten the stitch so that the thread can be fed to the feed roller 8 by the take-up lever and at the same time, the thread pulling operation by the take-up lever to tighten the stitch can be carried out. Such a control signal can be obtained easily by the rotary encoder. In FIGS. 7(b) and 7(c), reference numeral 11M represents the operation zone of the electromagnetic thread holding device 11a, 11b on the thread supply side and 7M represents the operation zone of the electromagnetic thread holding device 7 on the thread take-up lever side. According to this arrangement, sewing can also be carried out with satisfactory stitches in the high speed operation of the sewing machine. It is obvious that if the supply of the thread can be made without any trouble in the high speed operation, the supply of the necessary length of the thread for one stitch can be made naturally without any trouble for the low speed operation as represented in the diagram 7(c) showing the low speed operation.
In the thread feed period of the sewing machine described above, the pressure roller 10 is driven by the thread feed roller 8 through the thread 14 as shown in FIGS. 8(a), 8(b). In the thread feed stop period wherein the thread is gripped by the thread holding device 11a, 11b, the feed roller 8 is rotating but the movement of the thread 14 is stopped and the pressure roller 10 stops rotating as well.
However, if a plurality of threads having different feed lengths are pressed and fed by one pressure roller, slip develops unavoidably between the threads and the pressure roller 10. If this slip develops, the pressure roller is worn out due to friction with the threads and forms grooves so that the thread pressing operation might not be effected. Therefore, a pressure roller must be disposed for each of the threads as shown in FIG. 8(b).
When a large number of threads are fed, an independent pressure roller must be disposed for each of these threads as described above. If the number of threads becomes large, the structure gets more complicated and the production becomes more difficult. Unless the axis of the pressure roller 10 is in parallel with that of the thread feed roller 8, the rollers might come into mutual contact while gripping the thread and if they do, the pressure roller 10 might rotate while carrying the feed roller 8 even under the state where the thread is under the stop state. In consequence, the force of friction acts between the thread under the stop state and the pressure roller 10 and groove-like wear scratches are formed on the pressure roller 10 having low wear resistance resulting in an insufficient thread pressing operation.
Since the pressure roller rotates with the movement of the thread, a material having large frictional force such as a resin is used for the roller, but is likely to be worn out. In addition, the pressure roller should not absolutely rotate with the feed roller as the result of it coming into contact therewith. Accordingly, machining accuracy must be improved to insure parallelism between the feed roller and the pressure roller but it is very difficult to keep such parallelism. Japanese Patent Application No. 066595/1987 therefore proposes an invention of a thread feeding device wherein two threads are gripped between the thread feed roller and the pressure roller in the spaced-part relation with each other.
Furthermore, if the pressing force of the pressure roller to the thread feed roller is increased, the portion of the pressure roller pressing the thread undergoes deformation so that there is a limit to the increase of the pressing force. Since the inertia of the pressure roller must be reduced because it rotates and stops synchronously with the movement and stop of the thread, it has been necessary to use a light-weight material for the pressure roller and to reduce its rotary torque by use of bearings or the like.